This invention relates to microwave antennas in general and more particularly to an improved microwave antenna for use in Doppler navigation systems.
A common problem in Doppler navigation antennas is what is known as over-water shift. Because of the different characteristics of returned energy from land and water in the typical Doppler system, a shift occurs when flying over water which can lead to a considerable velocity error. One manner of overcoming this is what is known as a beam lobing technique in which each of the Doppler beams is alternated between two positions, a few degrees apart. Although such an approach has been found workable, it requires additional hardware and additional time.
Another approach is that disclosed in U.S. Pat. No. 2,983,920 granted to R. H. Rearwin and assigned to the same assignee as the present invention. Disclosed therein is a planar array of microwave antennas which are slanted at 45 degrees to permit generating a beam shape which exhibits a high degree of independence from over-water shift. However, the implementation disclosed therein is not particularly practical. U.S. Pat. No. 4,180,818 discloses the use of forward and backward firing slanted arrays to achieve frequency compensation. However, the use of slanted arrays creates other problems. Typically, an antenna aperture is bounded in a rectangular area. When a slanted antenna aperture is fitted into such a rectangular area, substantial areas of the rectangular area will not contain radiating elements. Thus the effective area and gain of the antenna are smaller than if the entire rectangular area were used.
In co-pending application Ser. No. 167,285, U.S. Pat. No. 4,347,516 assigned to the present assignee, an improvement to this prior art was made. The antenna disclosed therein solves the problems of the previously mentioned prior art by providing a rectangular antenna aperture which generates an antenna pattern very similar to the slanted aperture antenna. Although this antenna realizes the objectives of reducing over-water shifts and achieving frequency compensation while using the entire rectangular mounting area, frequency/temperature compensation occurs for individual beams of a multi-beam antenna output. Further, the single beam compensation technique of the co-pending application also has limitations in first side lobe level and wide angle sidelobes caused by the method of pattern generation therein.